This invention relates generally to implantable vasoocclusive devices for interventional therapeutic treatment or vascular surgery, and more particularly concerns a radiopaque super-elastic intravascular stent formed from a composite wire with enhanced radiopacity and increased corrosion resistance. The intravascular stent has superelastic or shape memory properties and improved radiopaque properties for visible detection under fluoroscopy, and the ends of the stent are flared radially outwardly to prevent radially and longitudinally inward deformation of the ends of the stent when the stent is stretched or disposed in a desired location in a patient's vasculature.
Vasoocclusive devices are therapeutic devices that are placed within the vasculature of the human body, typically via a catheter, either to block the flow of blood through a vessel making up that portion of the vasculature through the formation of an embolus or to form such an embolus within an aneurysm stemming from the vessel. The vasoocclusive devices can take a variety of configurations, and are generally formed of one or more elements that are larger in the deployed configuration than when they are within the delivery catheter prior to placement. One widely used vasoocclusive device is a helical wire coil having a deployed configuration that may be dimensioned to engage the walls of the vessels.
The vasoocclusive devices, which can have a primary shape of a coil of wire that is then formed into a more complex secondary shape, can be produced in such a way that they will pass through the lumen of a catheter in a linear shape and take on a complex shape as originally formed after being deployed into the area of interest, such as an aneurysm. A variety of detachment mechanisms to release the device from a pusher have been developed and are known in the art.
For treatment of areas of the small diameter vasculature such as a small artery or vein in the brain, for example, and for treatment of aneurysms and the like, microcoils formed of very small diameter wire are used in order to restrict, reinforce, or to occlude such small diameter areas of the vasculature. A variety of materials have been suggested for use in such microcoils, including nickel-titanium alloys, copper, stainless steel, platinum, tungsten, various plastics or the like, each of which offers certain benefits in various applications. Nickel-titanium alloys are particularly advantageous for the fabrication of such microcoils, in that they can have super-elastic or shape memory properties, and thus can be manufactured to easily fit into a linear portion of a catheter, but attain their originally formed, more complex shape when deployed. However, nickel-titanium alloy wires are also not radiopaque in small diameters, and a single nickel-titanium wire would need to be approximately 0.012 inches in diameter to be even slightly radiopaque. However, such a thickness of a single nickel-titanium wire would unfortunately also be relatively stiff and possibly traumatic to the placement site, particularly if used for treatment of delicate and already damaged areas of the small diameter vasculature such as an aneurysm in an artery or vein in the brain, for example.
One known type of stent includes a metal filament material formed of a metal outer member and an inner core formed of a different metal than the outer member. Another type of stent is formed of multiple filaments, each of which is a composite including a central core formed of a radiopaque and relatively ductile material such as tantalum or platinum allowing in vivo imaging of the stent, and an outer case formed of a relatively resilient material, such as a cobalt/chromium based alloy. An intermediate barrier layer of tantalum, niobium or platinum may be placed between the case and core, when the core and case materials would be incompatible if contiguous, due to a tendency to form intermetallics. A radiopaque case may surround the core, or to improve compatibility, a biocompatible cover layer, such as one or more of tantalum, platinum, iridium, stainless steel, niobium and titanium can surround the case.
Another type of endoprosthesis in the form of an elongated wire member is known that includes a central cylindrical or tubular core and an outer tubular sheath. An intermediate tubular layer may be disposed between the inner tubular layer and the outer tubular layer. The tube may include outer and inner layers formed of one material such as cobalt, carbon, manganese, silicon, phosphorus, sulfur, chromium, nickel, molybdenum, titanium, iron, alloys thereof and combination thereof, and an intermediate layer between the outer and inner layers formed of another material, such as gold, platinum, tantalum, iridium, tungsten, and alloys thereof and combination thereof.
Another type of stent preform includes an elongated metal core of a shape-memory alloy with a solid cross section, and a hollow outer sheath made of a biocompatible polymer such as a heat-shrinkable polymer material or polymer tape to prevent the core from directly contacting the body lumen. In another type of stent perform, an intermediate sleeve of a lubricious lining is disposed between the core and outer sheath.
Another type of stent is known that is made from multiple knitted or braided wire strands made of materials such as stainless steel, tungsten, titanium, nickel titanium alloy, gold or silver, coated on the outside with a biocompatible fluoropolymer.
While nickel-titanium wire such as nitinol wire has important shape memory and superelastic properties that are useful in vasoocclusive devices and stents, this material is not very radiopaque, so that it would be desirable to utilize a more radiopaque material that can be visualized under fluoroscopy. More radiopaque materials typically do not have shape memory and superelastic properties suitable for forming in vasoocclusive devices and stents, and combining such radiopaque materials with nickel-titanium wire such as nitinol wire are typically prone to galvanic corrosion, resulting in failure or compromise of the larger wire or the larger assembled system. It has also been found that when an intravascular stent is stretched longitudinally, the stent will naturally shrink in diameter, but will not shrink uniformly, in that the ends of the stent will commonly shrink in diameter to a greater extent than the diameter of a central body portion of the stent shrinks, resulting in a condition referred to as “fishmouthing” of the stent.
It would thus be desirable to provide an intravascular stent formed from a structural element that offers the advantages of a shape memory alloy such as a nickel-titanium alloy, and that incorporates radiopaque material, so that the intravascular stent can be visualized under fluoroscopy, and that is not subject to galvanic corrosion during use of the device. It would also be desirable to provide an intravascular stent that will resist radially and longitudinally inward deformation of the ends of the stent when the stent is stretched or disposed in a desired location in a patient's vasculature. The present invention meets these and other needs.